Methane for regenerating activated carbon

ABSTRACT

In a process where sulfur oxides have been removed from gas streams by adsorption onto activated carbon in the form of sulfuric acid, regeneration of the sulfuric acid laden activated carbon is accomplished in two stages by first contact the carbon at a temperature below 570* F. with hydrogen sulfide to form sulfur dioxide, which is recovered, and elemental sulfur which remains adsorbed on the carbon. The carbon with elemental sulfur adsorbed thereon is then contacted in the second stage with methane or natural gas at temperatures between from 800* to 1,300* F. to form hydrogen sulfide for the first stage and carbon disulfide. The addition of water, as steam, to the second stage converts the carbon disulfide to additional amounts of hydrogen sulfide for use in the first stage.

Johnson Feb. 1, 1972 METHANE FOR REGENERATING ACTIVATED CARBON Homer R.Johnson, Charleston, SC.

Westvaco Corporation, New York, NY.

Mar. 13, 1970 Inventor:

Assignee:

Filed:

Appl. No.:

US. Cl ..252/4ll R, 23/2 S, 23/178, 23/181, 23/226, 55/73, 252/411 SInt. Cl. ..B01d 15/06, B0lj 11/02 Field of Search ..252/411;23/2.1;55/73; 23/178, 181, 209, 226

References Cited UNITED STATES PATENTS Johswich ..55/73 X Kruel et a1...23/2

FOREIGN PATENTS OR APPLICATIONS 1,045,610 10/1966 Great Britain..252/4l1 Primary Examiner-Daniel E. Wyman Assistant Examiner-P. E.Konopka Attorney-Ernest B. Lipscomb and Robert S. Grimshaw [57] ABSTRACTIn a process where sulfur oxides have been removed from gas streams byadsorption onto activated carbon in the form of sulfuric acid,regeneration of the sulfuric acid laden activated carbon is accomplishedin two stages by first contact the carbon at a temperature below 570 F.with hydrogen sulfide to form sulfur dioxide, which is recovered, andelemental sulfur which remains adsorbed on the carbon. The carbon withelemental sulfur adsorbed thereon is then contacted in the second stagewith methane or natural gas at temperatures between from 800 to 1,300 F.to form hydrogen sulfide for the first stage and carbon disulfide. Theaddition of water, as steam, to the second stage converts the carbondisulfide to additional amounts of hydrogen sulfide for use in the firststage.

2 Claims, No Drawings M ETHANE FOR REGENERATING ACTIVATED CARBONBACKGROUND OF THE INVENTION This invention relates to a process forregenerating activated carbon for use in a sulfur oxide adsorbingprocess. More specifically, this invention relates to an improvedprocess for regenerating sulfuric acid laden activated carbon byreducing the sulfuric acid with hydrogen sulfide and then contactingwith methane or natural gas to eliminate use of the activated carbonadsorbent as a reductant.

In a dry process using activated carbon for removing low concentrationsof sulfur dioxide and sulfur trioxide from flue gas streams, such asfrom powerplants, the gas stream is contacted with an activated carbonin the adsorber where the sulfur dioxide is catalytically oxidized tosulfur trioxide and the sulfur trioxide is then hydrolyzed to sulfuricacid which adsorbs onto the activated carbon. In the normal combustionof fuels there are present in the flue gas amounts of oxygen and waterin excess of that needed to accomplish the oxidation of sulfur dioxideto sulfur trioxide and'the hydrolysis of sulfur trioxide to formsulfuric acid.

Several processes for regenerating the carbonaceous adsorbents have beenproposed, but each process is unsatisfactory for one reason or another.One process proposes the regeneration of sulfuric acid laden activatedcarbon by washing. The difficulty with this process is that largeamounts of water are needed, resulting in a voluminous stream of dilutesulfuric acid. Another proposed process is to heat the sulfur acid ladencarbonaceous adsorbent to form a concentrated stream of sulfur dioxideand carbon dioxide. A disadvantage of this regeneration process is thatthe carbonaceous adsorbent is used as the reducing agent therebyburning-off large amounts of carbonaceous material which must bereplaced.

An improved regeneration process is set forth in copending applicationSer. No. 752,298 filed Aug. I3, 1968. This regeneration processvirtually eliminates the difficulties of these earlier processes bycontacting in a two-stage process a sulfuric acid laden activated carbonadsorbent with hydrogen sulfide at elevated temperatures to form arecoverable stream of concentrated sulfur dioxide, i.e., 40-50 percent,eliminate carbon burn-off and leave elemental sulfur on the activatedcarbon adsorbed for treatment in the second stage. In the secondregeneration stage, hydrogen is reacted with the elemental sulfur toform hydrogen sulfide for use in the first stage. The activated carbonis thus completely regenerated for recycling producing desirablebyproducts without activated carbon burn-off.

This invention is an improvement on the second stage of theabove-described regeneration process. It is therefore a general objectof this invention to provide an improved process for removing sulfurdioxide and sulfur trioxide from gas streams thereby eliminating sulfuroxide air pollution. A more specific object is to provide an improvedprocess for regenerating sulfuric acid laden activated carbon. Anotherobject is to provide an improved two-stage process for regeneratingsulfuric acid laden activated carbon by contacting the carbon withhydrogen sulfide in the first stage to form sulfur dioxide and elementalsulfur, recovering the sulfur dioxide and subsequently contacting in thesecond stage the activated carbon adsorbent having elemental sulfuradhered thereto with methane or natural gas to convert the sulfur tohydrogen sulfide. A further object is to provide an improved economicalprocess for regenerating activated carbon without consuming the carbonduring regeneration. Further objects, features and advantages of thisinvention will be evident from the following disclosure.

SUMMARY OF THE INVENTION It has been found that methane or a methanecontaining gas, such as natural gas or methane producing hydrocarbon,may be used in place of hydrogen in the second stage of theabovedescribed two-stage regeneration process to react in an overallreaction with adsorbed sulfuric acid according to this reaction:

actlvated 480, E10 CO: carbon CH4 4H2SO4 DETAILED DESCRIPTION OF THEINVENTION Flue gas containing low concentrations of sulfur dioxide andsulfur trioxide is passed countercurrent to the continuously movingcarbon adsorbent and the sulfur oxides are adsorbed therefrom assulfuric acid. Physical adsorption of sulfur dioxide by activated carbonat flue gas temperatures is very low, but sulfur trioxide by comparisonis readily adsorbed in the presence of water. Satisfactory removal ofsulfur dioxide from gas streams depends upon the carbon acting as acatalyst in the oxidation to sulfur trioxide, which is hydrolyzed tosulfuric acid if water vapor is present in the flue gas. The oxygen andwater vapor necessary for the reaction are normally present in the fluegas, but they may be added if needed. The sulfuric acid thus formed isretained on the carbon surface and in this manner sulfur oxides areremoved from the gas stream.

activated (2) $02 A H2O H2304 The sulfur oxide removal efficiency of theprocess can be designed to be as high as required, for example, the gasstream as it passes to the atmosphere may be reduced to less than 5p.p.m. of sulfur from an initial concentration of 1,000 to 50,000 p.p.m.However, for economic reasons, it is usually preferable to adsorb aboutpercent of the sulfur oxides from the gas stream. One of the advantagesof an activated carbon process is that sulfur oxide adsorption may beperformed at flue gas temperature. Consequently, adsorption of sulfuroxides may take place at temperatures up to about 350 F. The preferredtemperature for sulfur oxide adsorption is between 200 and 260 F. inorder to retain buoyancy.

Any carbonaceous adsorbent may be used in the process of this invention,but an activated carbon is preferred. Since the regeneration proceduredoes not consume the activated carbon adsorbent, a highly activated,more effective hard carbon, such as those disclosed in copendingapplication Ser. No. 734,566, may be used. Through the ability to usethe more highly activated carbons, the throughput of gas per volume ofcarbon can be increased by five to seven times; compared to activatedchar. This achievement means a significant reduction in the size of theadsorption equipment required for a sulfur dioxide recovery process. Inaddition, hard activated carbons are much less subject to abrasion thanare chars, the result being lower attrition losses.

In the first stage, regeneration is efficiently carried out by usinghydrogen sulfide for recovering sulfur dioxide and sulfur from thesulfuric acid laden activated carbon. Although a number of reactions maybe postulated under the proper regeneration conditions, thestoichiometry necessary for practicing this invention is generallydescribed by the following reactions:

activated (3) H H: S

$02 S 2H2O carbon activated H280 3H, S

ambient temperature with the evolution of sulfur dioxide beginning atapproximately 350 F. and continuing upward. At temperatures below 350 F.reaction [4] predominates and sulfur remains adsorbed on the carbon. Asthe regeneration temperature is raised above 350 F. reaction [3] isfavored and the percentage of sulfur dioxide evolved is increase. Thisreaction is controlled to produce the requisite amount of elementalsulfur for conversion in stage two. It has been shown that althoughregeneration may be carried out at temperatures above 570 F., completesulfur trioxide and sulfuric acid reduction may be accomplished attemperatures below 570 F. The sulfur dioxide formed is continuouslypurged and conveyed from the regenerator to further processing for saleas indicatedabove.

Elemental sulfur is then removed from the activated carbon in the secondstage of regeneration by direct reaction with methane or a methanecontaining gas, such as natural gas or methane producing hydrocarbons,according to reaction [5]:

activated CH4 48 GS: ZHzS rb n e The hydrogen sulfide formed by thisreaction is carried to the first regenerator for use in reactions [3]and [4]. The thus completely regenerated activated carbon adsorbent isthen recycled without any loss in effectiveness for adsorption of sulfuroxides. It is desirable to carry out reaction [5] at temperatures above500 F. It has been found that temperatures between 800 F. and 1,300 F.satisfactorily remove the sulfur at a sufficiently short reaction time,but lower temperatures may be used if sufficient reaction time isallowed. The complete regeneration thus encompasses the chemicalreduction of adsorbed sulfur trioxide or sulfuric acid to sulfur dioxideand adsorbed elemental sulfur by hydrogen sulfide and the subsequentreaction of adsorbed elemental sulfur with methane to form hydrogensulfide.

If the adsorbed sulfur is removed according to reaction [5], the amountol'hydrogen sulfide is less than that required in the first stage ofregeneration. The amount of hydrogen sulfide necessary for completereduction of sulfuric acid in stage one may be supplied from an externalsource or it is possible to convert a sufficient amount of the carbondisulfide formed during reaction [5] by injecting steam along with themethane according to reaction [6]:

activated car on The equilibrium conversion is almost complete in thetemperature range from 260 to l,300 F., preferably about 800 F.Activated charcoal give a conversion of 83 percent at 748 F. and a spacevelocity of 850 bed volumes per hour. The amount of water vapornecessary to give maximum conversion need be only slightly above thetheoretical amount. By mixing water vapor, methane and sulfur vapor, andpassing the mixture through the activated carbon bed hydrogen sulfidecan be obtained in one reaction zone instead of having two zonesoperating at different temperatures. This procedure would not bedesirable if the steam and/or carbon dioxide react with the activatedcarbon; however, at the temperatures of regeneration, these reactions donot proceed to an appreciable extent.

in evaluating the methane requirement for the second stage ofregeneration, it is considered that methane reacts with adsorbed sulfuraccording to equation and that the carbon disulfide formed by thisreaction is converted to hydrogen sulfide and carbon dioxide accordingto reaction [6]. Thus, the overall reaction for the second stage ofregeneration can be written as:

activated (3 n) I o (9) on mso. so,

Examination of reaction [9] indicates that n can range from l to 3 withthe product distribution and the methane requirement depending upon thevalue of n. A, value of l for n results in the minimum methanerequirement and sulfur dioxide for the final product. For this value ofn, the CH :H SO ratio is l /4: 1. If hydrogen sulfide is the desiredproduct, the value of n is 3 which results in a CH :H SQ,.ratio of 1:1.The methane requirement is the same whether steam reformationis usedtoproduce hydrogen or whether methane is used ina direct reaction withadsorbed sulfur. Of course, if methane can be used directly, thereforming equipment is not required.

The advantages offered by this invention include the ability to use aprocess whereby burn-off of activated carbon may be virtually eliminatedand methane reforming equipment is not required. Since carbon burn-offis eliminated, a second advantage is the ability to use an activatedcarbon possessing superior adsorption-rate and capacity characteristics.Regeneration by chemical reduction has the further advantage ofproducing a stream concentrated in sulfur dioxide which can be furtherprocessed to such products as liquid sulfur dioxide, sulfuric acid orsulfur. The adsorption and regeneration may be carried out in a fixedbed system of a continuously moving bed system, but the preferredprocess utilizes a fluidized bed system.

The practice of this invention may clearly be seen in the followingexamples.

EXAMPLE 1 Experiments were conducted with a coal-based activated carbonhaving a mesh size of [4X40 containing adsorbed sulfur to study theeffect of temperature upon sulfur removal for .temperatures ranging from800 to l,300 F. during the second stage of regeneration. These runs weremade in a linch-diameter, fixed bed, regenerator at various spacevelocities using an inlet gas'stream containing 45% CH,/55% He. Theconditions and results of the runs according to reaction [5] are givenin the table.

EFFECT OF TEMPERATURE UPON SULFUR REMOVAL USING METHANE IN CARBONREGENERATION Sulfur content of carbon Space Before After velocityMaximum regen regen. (vol. fiisl bed temper- (weight (weight vol. 0-ature F.) percent) percent) EXAMPLE 2 The effect of space velocity at atemperature of l,200 F. was investigated. it was felt that acceptablesulfur removal could be obtained at this temperature with only a smallamount of methane being cracked. A l-inch-diameter, fixed bed, quartzreactor was used. Three runs were made with a coal-based activatedcarbon containing 10 percent adsorbed sulfur using an inlet stream of45% CH /55% He. The heating rate for these runs was programmed such thatthe maximum bed temperature, 1,200 F., was reached in 3 hours and thistemperature was maintained for 1 additional hour. The conditions andresuits of the runs are given in the table.

Space Sulfur content velocity after regenera- Recovered (vol. gas/ tion(weight sulfur Run N vol. c-hr.) percent) as H23 The results given inthe above table indicate that the sulfur content is reduced to about 1.7percent and that this value does not depend upon space velocity for therange studied. It is also indicated that approximately 80 percent of thesulfur removed was recovered as hydrogen sulfide and that the percentageincreases slightly with increasing space velocity.

EXAMPLE 3 The effect of the addition of steam to the methane reactionwas tried using the equipment of example 1. A stream of regeneration gascomposed of 40% CH 10% steam, and 50% He was contacted with an activatedcarbon containing l0 percent adsorbed sulfur at a temperature of l,200F. and a space velocity of 100 bed volumes per hour. The results showedthat the sulfur content was reduced below l.8 percent. Also,approximately 92.0 percent of the sulfur removed was recovered ashydrogen sulfide. This shows that all the hydrogen sulfide necessary forreduction of sulfuric acid in stage one may be supplied in the secondstage. The remaining sulfur was removed in the form of sulfur dioxide,COS and CS While the invention has been described and illustrated hereinby references to various specific materials, procedures and examples, itis understood that the invention is not restricted to the particularmaterials, combinations of materials, and procedures selected for thatpurpose. Numerous variations of such details can be employed, as will beappreciated by those skilled in the art.

lclaim:

1. In a two-stage process for regenerating a sulfuric acid ladencarbonaceous adsorbent by first contacting said adsorbent with hydrogensulfide to reduce said sulfuric acid to a member of the group consistingessentially of sulfur dioxide, elemental sulfur and mixtures thereof,recovering said sulfur dioxide while retaining said elemental sulfur onsaid adsorbent and second, contacting said carbonaceous adsorbentcontaining elemental sulfur adsorbed thereon with a gas from the groupconsisting essentially of methane and natural gas at a temperaturebetween 500 and 1,300" F. to reduce said elemental sulfur to hydrogensulfide and carbon disulfide, the improvement comprising, simultaneouslywith said second stage contacting said adsorbent with steam to convertsaid carbon disulfide to hydrogen sulfide for use in said first stageand carbon dioxide.

2. The process of claim 1 wherein said second stage is carried out at atemperature between 800 and l,300 F.

2. The process of claim 1 wherein said second stage is carried out at atemperature between 800* and 1,300* F.